1. Technical Field
The present invention relates to backflow preventers and, more particularly, to a backflow preventer in which check valves are provided at the upstream and downstream sides, respectively, and a relief valve is provided in an intermediate chamber formed between the two check valves.
2. Background Art
As a conventional backflow preventer for use in the piping of a water supply system or the like, Japanese Patent Application Publication No. Hei 11-153300, for example, discloses a backflow preventer of pressure-reducing type in which check valves are provided at the upstream and downstream sides, respectively, and a relief valve is provided in an intermediate chamber formed between the two check valves. In this type of backflow preventer, the valving element of the relief valve is constantly acted upon by a spring in the direction for opening the valve. A valve stem extending from the body of the valving element is secured to a diaphragm. The diaphragm is acted upon at both sides thereof by the primary pressure and the intermediate-chamber pressure, respectively. The differential pressure between the two water pressures acts in the direction for closing the relief valve. Thus, the relief valve is normally closed by the action of the differential pressure. The relief valve is opened when the difference between the primary pressure and the intermediate-chamber pressure has become smaller than a predetermined value, for example, when sealing of the upstream check valve cannot be properly performed owing to an abnormality in the check valve, or when there is reduction in the upstream pressure due to the burst of the piping or the like. By opening the relief valve, water is discharged from the intermediate chamber to prevent backflow.
Check valves usually used in the above-described backflow preventer are of the type that a valving element that is movable in the flow direction is urged toward a valve seat by a spring or other urging member. When the backflow preventer is installed in the piping of a water supply system, the relief valve is open in a state where water has not yet been supplied therethrough. Therefore, when the supply of water is started for the first time, the supplied water will be undesirably discharged from the intermediate chamber unless the relief valve is closed before the first check valve opens. Accordingly, a fairly strong spring is used as the spring of the first check valve to prevent the first check valve from immediately opening when the supply of water is started for the first time. In the meantime, the water pressure at the upstream side of the diaphragm is introduced to act on the high-pressure side of the diaphragm, thereby allowing the relief valve to be surely closed before the first check valve opens. Further, the spring of the relief valve is set so that the relief valve is opened when the differential pressure reaches a set value, as has been stated above. Regarding the first check valve, the spring used therefor needs to be set so as to generate a differential pressure that can surely keep the relief valve closed against the spring of the relief valve. During the supply of water, it is also necessary to keep the relief valve closed by a differential pressure between the upstream and downstream sides. Therefore, the first check valve needs to generate a correspondingly large differential pressure. Due to these conditions to be met, considerably large pressure losses are produced in the conventional pressure-reducing backflow preventer.
A booster pump is used to supply water to the upper floors of a building. The pump has to do extra work correspondingly to pressure losses produced by pipes, valves, backflow preventer, etc. connected to the upstream and downstream sides of the pump. A large pressure loss is produced particularly in the first check valve of the backflow preventer, as has been stated above, which places a heavy load on the pump. Accordingly, if pressure losses due to backflow prevention are reduced, the load on the pump can be reduced to a considerable extent, and the power consumption can be reduced.
Standards are provided for backflow preventers. Among them is the pressure difference between the upstream and downstream (i.e. intermediate chamber) sides of the first check valve which will cause the relief valve to open. In this regard, if the pressure loss in the first check valve is reduced, the difference between the primary pressure and the intermediate-chamber pressure during the water supply may become smaller than the specified pressure difference, resulting in the relief valve undesirably opening during the water supply.